Electronic components such as semiconductor chips are often very small and have minimal gaps between connectors such as pins. Conventional solder may give rise to difficulties because the solder may bridge the gap between two pins. Therefore anisotropically-conductive adhesives have been proposed for electrical interconnection. An anisotropically conductive adhesive (ACA) conducts electricity in one direction only (usually denoted as the Z direction) and should eliminate conduction in the plane perpendicular thereto (the X and Y directions).
Various proposals for ACA's are reviewed by Ogunjimi et al. in Journal of Electronics Manufacturing (1992) 2, 109-118. They usually consist of an adhesive matrix in which conductive particles are dispersed. The particles may be metal particles or non-conductive particles (e.g. plastic or glass) with a thin metal coat. After the adhesive has been applied between two conductors, bond line thickness may then be reduced by pressure applied during cure so that the particles in the adhesive contact the two conductors but do not contact one another laterally (see U.S. Pat. No. 4,740,657 Tsukagoshi et al.). Alternatively, conductive particles which are also magnetic may be aligned by use of a magnetic field so that they form a chain and provide an anisotropically conductive path along the direction of the field. The adhesive is then cured while the field is applied (see U.S. Pat. Nos. 3,359,145 Salyer et al; 4,548,862 Hartman; 4,644,101 Jin et al; and 4,170,677 Hutcheson). U.S. Pat. No. 4,737,112 Jin et al. uses single-particle bridging with essentially uniform distribution resulting from application of a magnetic field. Particles are magnetized N-S by the magnetic field, resulting in lateral repulsion between particles. The text at column 4 lines 6-8 suggests that the particles may have a non-magnetic, non-conductive core portion which is coated with a magnetic conductive coating. However no working Examples of the use of such particles are described. The Examples in the Jin et al. patent use gold coated nickel spheres which would have a solid core of magnetic material.
In an unrelated area of technology, it is known to make a magnetic liquid or "ferrofluid" consisting of a colloidal suspension of minute ferromagnetic particles in an non-magnetic carrier liquid. A typical ferrofluid may consist of magnetite particles (Fe.sub.3 O.sub.4) having a particle size in the range 2 nanometers to 0.1 micrometers (and a mean size of about 0.01 micrometers) in kerosene as carrier liquid with a surfactant to prevent agglomeration of the particles (see Skjeltorp "One- and Two-Dimensional Crystallization of Magnetic Holes" in Physical Review Letters, Volume 51, Number 25, Dec. 19, 1983, 2306-2309, the contents of which are incorporated by reference). Skjeltorp describes the production of "magnetic holes" inside a thin layer of magnetic fluid containing a monolayer of polydisperse polystyrene spheres with diameters in the micrometer range. U.S. Pat. No. 4,816,988 (Skjeltorp) describes a method for bringing bodies immersed in liquid to form regular structural patterns by dispersing non-magnetic, essentially monodisperse, particles having uniform sizes and shapes in a ferrofluid so that the particles create non-magnetic "holes" in the ferrofluid, and applying a substantially homogeneous magnetic field to the ferrofluid. Each of the dispersed non-magnetic particle bodies then assumes a magnetic moment corresponding to the volume of liquid displaced by the body, but inversely directed. Magnetic interaction forces then prevail between the particle bodies, which may thus be collectively controlled by the external magnetic field to assume structural patterns. When the particle bodies are relatively large (greater than or equal to 5 micrometers) compared to the size of the magnetite particles (of the order of 0.01 micrometers) within the ferrofluid, they undergo negligible Brownian motion. However when the particles are smaller than about one micrometer, Brownian motion introduces fluctuations into the system which can prevent the build up of very long chains and cause chain pieces to reptate (Skjeltrop A. T. and Helgesen, G. Phyisica A, 176, 37, 1991; Skjeltrop A. T. J. Appl. Physics 57(1), 3285, 1985). Nevertheless with small particle body inclusions it is still possible to develop longer and stiffer chains by increasing the magnetic field. The utility of Skjeltrop's invention in U.S. Pat. No. 4,846,988 is to form patterns which may influence electromagnetic and acoustic waves, simulate states and processes in atomic or molecular structures and the like. Skjeltorp states that the non-magnetic particle bodies are mondisperse bodies (i.e. a great number of bodies have essentially identical size and form) and are preferably made of plastic material, in particular polystyrene. There is no suggestion of using electrically conductive particle bodies.
Neither is there a suggestion that pure noble metal colloids, with particle sizes comparable to those of the magnetic material itself, can be used to form anisotropic structural patterns made up of metallic pathways by first using magnetic field induced aggregation of the noble metal and second aligning said aggregates. It is known, for example, that gold and other noble metals can be made in colloidal form in an aqueous or non-aqueous state (Nakao Y., J Chem Soc Chem Commun., 826, 1993, Nakao, Y. and Kaeriyama K., J. Colloid Interface Sci., 110(1), 82, 1986), and that colloidal metal particles may be dispersed in polymerisable systems such as acrylics, styrenes and acrylonitrile (Cardenas-Trivino G. et al., Chemistry of Materials, 1, 481, 1989, Polymer Bulletin 27, 383, 1992, Polymer Bulletin 26, 611, 1991, Polymer Bulletin 31, 23, 1993; Nakao et al. loc cit.). Still further, it is known to be possible to produce so-called ferrofluid composites, which differ from stable co-colloidal systems but none the less comprise minute metallic components which align in response to a magnetic field (Popplewell, J. et al. J.Magnetism & Magnetic Materials, 54-57, 761, 1986; see also Kopcansky, P., et al. Acta Phys Slov. 39(4), 259, 1989). The latter systems have been proposed as possible polarisers or attenuators for microwave (3 mm wavelength range) radiation. There has been no suggestion in the literature that such systems could be rendered permanent following the removal of the magnetic field. The possibility that co-colloidal systems could undergo magnetic field induced phase separation followed by alignment of metal aggregates in structural patterns which can be subsequently locked permanently in position and be used as an anisotropically conductive adhesive, has not been suggested.
U.S. Pat. No. 5,075,034 Wanthal describes a two component adhesive composition which is curable by induction heating (i.e. with an induced magnetic field) and which contains conductive carbon black along with iron oxide particles. However there is no suggestion that the iron oxide particles may be of such small particle size as to form a colloidal suspension. This patent therefore does not relate to the field of ferrofluids or of anisotropically conductive adhesives.
In a further unrelated area of technology, U.S. Pat. No. 4,946,613 Ishikawa describes a photosetting ferrofluid for use in magnetic flaw detection or for visualising magnetically recorded patterns. The photosetting ferrofluid comprises a carrier, a ferrofluid in which the ferromagnetic particles have an adsorbed surfactant (or the surfactant is dispersed in the carrier) and a photosetting resin. The photosetting resin may be the carrier. The ferrofluid is applied to a surface to be analysed and is then subjected to a magnetic field. The applied ferrofluid will be attracted to the portion where the magnetic flux leaks i.e. to cracks or defects in the surface, and will swell to form a pattern corresponding to the configuration of the defect portion. A beam of light is then used to set or harden the photosetting resin so as to fix the defect pattern thus formed.
Ishikawa does not envisage the application of a magnetic field to create a chosen alignment of particles, followed by fixation of this alignment.
ACA's rendered anisotropic by application of a magnetic field have not been adopted commercially, so far as the present Applicants are aware. The prior art proposals (e.g. as in U.S. Pat. Nos. 4,548,862 and 4,644,101) require specialised magnetic particles which are electrically conductive. Such particles are expensive and difficult to obtain.
In addition, magnetic particles which have been aligned by a magnetic field are likely to be randomly distributed when viewed in a plane transverse to the alignment. This is undesirable for interconnection in the electronics field, where the distribution of conductive pathways is critical in order to ensure conduction between each opposed pair of conductors.